Exporting Docker images from Dagger

• August 11, 2023
• docker  
• dagger  
• containers  

I started using Dagger this week, and if you have any sort of build and test system based on shell scripts and Dockerfiles, Dagger will be a big improvement.

This post documents how to export a container image that you build in Dagger to your local Docker instance. This process is described in the Dagger documentation but I needed to go one step further and tag the image that I exported.

The first step is to actually export the container image from Dagger. This will write an OCI tarball to the local filesystem.

var container *dagger.Container

ok, err := container.Export("path/to/container-image.tgz")
if err != nil {
        log.Fatal(err.Error())
}
if !ok {
        log.Fatal("container export failed")
}

Now you have tarball containing an OCI, image, you have to import it into Docker using “docker load”, not “docker import” (see #3710 for example). However, “docker import” doesn’t let you tag the image at import time, which is awkward because no-one wants to refer to images just by their ID.

buildImagePath := "path/to/container-image.tgz"
cmdImport := exec.Command("docker", "load", "--quiet", "--input", buildImagePath)
out, err := cmdImport.CombinedOutput()
if err != nil {
        log.Fatalf("failed to export builder image: %s", err.Error())
}

Note that in the command above, I used the --quiet flag. When this is set, Docker will write a single lin eof output containing the hash of the image that it imported. We can scrape the container ID from this output and use it to tag the image that we just created.

r := regexp.MustCompile("Loaded image ID: ([:a-z0-9]+)")
matches := r.FindStringSubmatch(string(out))
if len(matches) == 0 {
    log.Fatalf("failed to match image ID from Docker output %q", string(out))
}

repoName := "container-image:latest"
cmdTag := exec.Command("docker", "tag", matches[1], repoName)
if out, err := cmdTag.CombinedOutput(); err != nil {
    log.Fatalf("failed to tag builder image: %s", string(out))
}

log.Printf("tagged build container image %q as %q", matches[1], repoName)